This invention is in the field of coatings, in particular, water reducible coatings. For purposes of this application, a water reducible coating is defined to be a colloidal dispersion in an organic solvent and water whose resin concentration can be reduced with water or solvent, which when reduced with a coalescing solvent forms a durable coating when applied to a substrate surface. Other names applied to water reducible coatings are waterborne, water solubilized, and water-dilutable. This application relates to a particular resin formulated to be water reducible and to give good adhesion and water resistance to the final coating.
Generally, coating resins are insoluble in water. Therefore, in general practice they have been dissolved in a suitable organic solvent or dispersed in water with the aid of emulsifying or surfactant agents in order to provide a coating composition suitable for application. A serious disadvantage of organic solvent solutions is their inherent potential toxicity, flammability, and environmental pollution.
The Clean Air Act of 1970 constitutes the basis of air pollution control regulations. It provided that the Environmental Protection Agency should issue National Ambient Air Standards, which has been done. These standards provide for limitations on particulate matter, sulfur dioxide, carbon monoxide, oxides of nitrogen, hydrocarbons, and photochemical oxidants. The last two of of these, arising from coating solvents, are the major concerns to the coating industry. Many state implementation plans under the Clean Air Act have followed the general framework of the Rule 66, which was adopted by Los Angeles County, California, in 1966 and which greatly limits these latter two contaminants.
These regulations are generally addressed to the users of coatings rather than the manufacturer. Coatings manufacture can in general be conducted with little or no emission of solvents into the atmosphere, but the application and curing of these coatings on finished products such as automobiles and refrigerators require the evaporation of large amounts of solvent and thus fall under these regulations.
The most common systems being developed by coatings companies to meet these regulations are waterborne, powder, electrodeposition, high solids, conforming solvents, and polymer-monomer blends for ultraviolet light cure type of coatings. It is expected that water reducible or water borne coatings will have a major share of the coatings market in the future. Although very few waterborne coatings are devoid of organic solvents, several of the state regulations have exempted water-thinned coatings which contain 20% or less volatile material.
Aqueous dispersion or latex-type coatings contain high molecular weight polymers dispersed as an emulsion in an aqueous medium. Generally, acrylic latex coatings exhibit excellent toughness, good chemical and water resistance and excellent durability. However, the use in industrial coatings has been limited because of certain disadvantages (external thickener required for viscosity control, poor pigment dispersability, low gloss, foaming, complex formulation, and substrate wetting problems). Also, such water dispersions, when used to prepare coatings, generally require additional hardeners to overcome residual emulsifiers and curatives to crosslink the resin where water insoluble coatings are desired. Otherwise, such coatings prepared from a water dispersion would simply bleed away with a water wash.
On the other hand, water reducible polymers can be considered to be ultrafine particles of a molecular weight intermediate between that of emulsion polymers and true solutions. These polymers contain polar groups, which impart a degree of solubility. They are hybrids of emulsion polymers and solution polymers since they possess properties characteristic of both types. The affinity of the resin for water is controlled by the extent of solvation of the polar groups (carboxylic acid moieties in the case of acrylic polymers) in the polymer backbone. Solvation is achieved by pH adjustment and/or the addition of a water miscible polar cosolvent. Water reducible resins can be alkali soluble (containing acidic groups), acid soluble (containing basic groups), or nonionic (containing amide or hydroxyl groups in conjunction with either acid or basic components). The resins of this invention are of the acidic type or ionic.
Coatings made from the water-reducible resins have advantages over emulsion type coatings. They have better flow and leveling characteristics. Pigment dispersibility is easier. Formulation of the coating composition is not as complex, and gloss of the final coating is higher. Another important benefit of the water reducible type of coating is that it may be formed without substantial amounts of surfactant or emulsifying agents, which in large amounts can inhibit the physical properties of the final coating.
Representative of the types of resins presently utilized in water reducible coatings are: alkyd (e.g. U.S. Pat. No. 4,051,089), styrene acrylics, acrylics, and polyesters. This invention is concerned with the acrylic type of resin. Two U.S. Patents on the subject of acrylic water reducible resins are: U.S. Pat. No. 3,862,071 (acrylate-acrylic acid copolymer and metallic flake pigment) and U.S. Pat. No. 3,904,569 (aqueous dispersion containing a carboxylic acid-containing resin and an aliphatic polyamine). Acrylic water reducible resins having both hard and soft segments and to which a plasticizer is added are disclosed in U.S. Pat. No. 4,064,092 of which there are two divisional applications on file (Ser. No. 827,196 and 827,037 now allowed, both filed Aug. 23, 1977).
Adhesion and water resistance have been problems with water reducible resins. Such resins have an affinity for water and coatings derived from them swell upon contact with water. Adhesion may be improved by the addition of plasticizer (e.g. a compatible phthalate), but some plasticizers can be extracted with water or soapy water.
There are various monomers which are known adhesion promoters: vinyl pyridine, 2-ethylhexyl methacrylate, methacrylic acid, dimethyl amino ethyl methacrylate, and isobutoxymethacrylamide. N-vinyl-2-pyrrolidone is known to be a hydrophilic, adhesion promoting monomer. It is known that when N-vinyl-2-pyrrolidone comprises 1 to 20 percent of the monomeric units in a polymer, it can confer characteristics such as strength, dye receptivity and hardness to the polymer. It is also known that N-vinyl 2-pyrrolidone can modify water-swelling equilibrium, melting point, dielectric properties or surface activity. Vinyl pyrrolidone copolymers are also known to give smooth, continuous films with good hardness, stiffness and grease resistance properties.
The research and development which led to this invention were part of an effort to obtain water reducible acrylate coating resins with unusually good adhesion to metal surfaces and water resistance to prevent powdering and cracking. It has been found that the incorporation of from about 0.5 to about 5 percent N-vinyl-2-pyrrolidone into a specific class of acrylate polymers achieves this object better than the other adhesion promoters which were tested.